An imaging device, or sensor, is a photosensitive electronic component used to convert electromagnetic radiation into an analog electrical signal. This signal is then amplified and digitized by an analog-to-digital converter and finally processed in order to obtain a digital image.
The imaging device employs the photoelectric effect. An imaging device generally comprises photosites arranged in a matrix, each photosite corresponding to a pixel of an image.
A photosite comprises at least one photosensitive zone, especially a photodiode, and a zone where charge accumulated in the photodiode is read. Photons captured by a photodiode of a photosite are converted into electron/hole pairs. Charge of a first type, for example holes, created in the photosensitive zones is drained depthwise towards the substrate (p+) and charge of a second type, for example electrons, is stored in the photosite before being read by virtue of an electronic system.
Generally, this electronic system, which controls the photodiode, comprises, especially when the photodiode is a fully depleted photodiode, a transfer transistor that controls the transfer of charge stored in the photodiode to a zone where the charge is read. This charge read zone forms a sensing node to which conventional read electronics are connected, especially comprising a read transistor.
A photodiode functions in a cycle comprising at least one integration step, one measurement step, and one reset step. The integration step corresponds to the photogeneration of charge and its accumulation during exposure of the photodiode to light. The measurement step corresponds to the generation of a signal depending on the amount of photogenerated charge accumulated in the photodiode. The reset step corresponds to the removal of the photogenerated charge.
In FIGS. 1a to 1c, a photosite 1 according to the prior art is schematically represented. FIG. 1a schematically illustrates a top view of the photosite 1, FIG. 1b shows a cross-sectional view through the photosite 1 in FIG. 1a in the plane B-B′, and FIG. 1c shows a cross-sectional view through the photosite 1 in FIG. 1a in the plane C-C′.
The photosite 1 comprises, in a semiconductor substrate, a vertically confined photodiode 2. The photosite 1 is bounded, in the substrate, by trench isolation 3.
The photodiode 2 comprises a charge storage zone defined by an n-type semiconductor well Nw produced in an intermediate p-type semiconductor region Pw lying vertically between a lower p-type semiconductor zone Pinf and an upper p-type semiconductor zone Psup. The photosite 1 comprises a charge read zone SN placed opposite the charge storage zone of the photodiode 2 on the surface of the substrate. The photodiode 2 is a pinned photodiode in that it comprises an n-type charge storage zone Nw lying between two p-type zones, in this case the intermediate semiconductor zones Pw, the upper semiconductor zone Psup and the lower semiconductor zone Pinf. The pinning of the photodiode 2 allows a vertical potential well to be created in the charge storage zone Nw, which well is completely depleted of free carriers, allowing photogenerated charge to be accumulated in the charge storage zone Nw before said charge is transferred to the charge read zone SN of the photosite 1.
The photodiode 2 comprises a zone for collecting charge, which zone is separated from trench isolation 3 by a p-type passivation zone 4, the dopant concentration of which is much higher than the dopant concentration in the p-type zones of the collection zone. The passivation zone 4 also separates the lower semiconductor zone Pinf from the lower surface, or back side Ar, of the photosite 1.
Passivation of the upper surface, or front side Av, of the photodiode 2, is achieved by a surface passivation 5 that also possesses a much higher dopant concentration than the dopant concentration in the p-type zones of the collection zone.
The photosite 1 also comprises a charge transfer transistor TG overhanging the well Nw, enabling the transfer of charge to the read zone SN, and a follower transistor SF, allowing the signal measured by the read zone SN to be amplified. The photosite 1 also comprises means 6 for isolating the read node SN, which is placed between the well Nw and the isolating means 6, a passivation well 7 placed under the read node SN, and two isolating zones 8 lying between the isolating means 6 and the trench isolation 3 so as to isolate the follower transistor SF from the charge transfer transistor TG.
The semiconductor well Nw forming the charge storage zone is laterally pinned in a direction parallel to the surface of the semiconductor substrate. This is done in order to increase the amount of charge that the photodiode 2 can store, by increasing the dopant concentration. Lateral pinning of the charge storage zone Nw reduces the width of the well to be implanted. However, the smaller the width of the well Nw, the harder it becomes to produce a charge storage zone Nw right through the entire depth of the substrate.
Consequently, part of the charge collection zone does not contain a potential well allowing photogenerated charge to be stored. The charge created in the parts with no potential well is very unlikely to recombine in the charge collection zone. This charge may then drift until it passes into a passivation zone 4 or 5 in which the recombination probability is higher. The loss of charge due to recombination degrades the performance of the photodiode 2.
In particular, in the case where the photosite is back-lit, via the back side Ar, photons, especially short-wavelength photons such as blue photons, create charge at small depths in the substrate, i.e. just below the surface of the back side Ar. This charge is therefore created at a greater distance from the charge storage zone than the charge created by photons of longer wavelength, such as red photons, which generate charge deeper in the substrate. The charge created by blue photons is more likely to be lost to electronic recombination in a passivation zone than the charge created by red photons.